Various conventional lighting units, such as vehicle lighting units, have been known which can change the travelling direction of light from a light source, for example, vertically oriented, to be spread horizontally wide by a light guide body. This configuration can provide a horizontally elongated lighting unit that can project light through the elongated front surface of the light guide body such that the elongated light guide body is illuminated with the light in a line shape. This type of lighting unit can include those disclosed in Japanese Patent Application Laid-Open No. 2006-339121 (or US20060274621A1), for example.
FIGS. 1A and 1B illustrate one example of this conventional type of lighting unit. As illustrated in the drawings, the lighting unit can include a light source 8 oriented upward and a light guide body 9 configured to control the light emitted from the light source 8. The light guide body 9 can include a light incident portion 90 that faces to the light source 8, a first reflecting surface 91 and a pair of second reflecting surfaces 92, 92 that are located above the light incident portion 90, a pair of third reflecting surfaces 93, 93 that are located on respective sides of the lighting unit to correspond to the pair of second reflecting surfaces 92, 92, and a front surface 94 that is horizontally elongated. Among them, the first reflecting surface 91 can be located at a front position above the light incident portion 90, and can be configured to be an inclined plane directed forward and upward by an angle of 45 degrees. The pair of second reflecting surfaces 92, 92 can be configured to be bilaterally symmetric with respect to the horizontal center of the light incident portion 90. The pair of second reflecting surfaces 92, 92 can be configured to be an inclined plane directed forward and leftward or rightward, respectively, by an angle of 45 degrees.
In the lighting unit with this configuration, as illustrated in FIG. 1B, the light emitted from the light source 8 can be first incident on the light incident portion 90 of the light guide body 9 to enter the same while the light can be collimated. Part of the light on the front side can impinge on the first reflecting surface 91 and be reflected by the same forward while almost all the remaining part of the light can impinge on the pair of second reflecting surfaces 92, 92 and be reflected by the same and divided rightward and leftward. After that, the divided traveling light rays can impinge on the pair of third reflecting surfaces 93, 93 so as to be reflected by the same forward. The reflected light can be projected through the front surface 94 elongated in the right-to-left direction, thereby being projected forward while the front surface 94 can be illuminated with light as if it is a line-shaped lighting unit.
With the above-described conventional lighting unit, however, it is difficult to configure a more elongated front surface 94 of the light guide body 9 illuminated with light. This may be because the pair of second reflecting surfaces 92, 92 of the light guide body 9 can only reflect the light that has travelled from the light incident portion 90 while collimating the light.
A detailed description of the problems in association with the conventional lighting unit will be given with reference to FIGS. 2A and 2B. Even if the light guide body 9 is configured to make a front surface 94 longer by simply extending the front portion of the light guide body 9, the front portions of the third reflecting surfaces 93, 93 located forward of the second reflecting surfaces 92, 92 cannot receive the light as illustrated in FIG. 2A, whereby both ends of the front surface 94 in front of the respective front portions of the third reflecting surfaces 93, 93 become dark.
To cope with this defect, as illustrated in FIG. 2B, the positions of the first reflecting surface 91 and the pair of second reflecting surfaces 92, 92 as a whole can be moved forward. In this case, however, the rear portions of the third reflecting surfaces 93, 93 located rearward of the second reflecting surfaces 92, 92 cannot receive light. As a result, the inner portions of the front surface 94 in front of the rear portions of the third reflecting surfaces 93, 93 become dark.
If the light incident portion 90 is enlarged and the corresponding first reflecting surface 91 and the pair of second reflecting surfaces 92, 92 are also enlarged, the occurrence of the dark portions can be prevented. In this case, however, the light guide body 9 must be sized up, in particular, in the vertical direction, resulting in various other problems arising, such as deteriorated moldability, attenuation of light due to extended optical path length, etc. Thus, this countermeasure is not preferable.